Such devices, intended to be able to operate with unpolarized light, have already been described in the literature. Notably, the patent application WO 2008/091896 may be mentioned, which describes a lens consisting of an electrode with a kinoform profile for forming a cell which is filled with a cholesteric liquid crystal, or the French patent FR 10 01114, describing a phase modulator based on a liquid crystal with a helical structure.
In general, one of the main characteristics of liquid-crystal cells is the fact that it is possible to change the optical index of the medium by applying an electric field. Specifically, the liquid-crystal molecules, notably nematic molecules, have an ordinary optical axis and an extraordinary optical axis. These molecules can be orientated under the action of an applied electric field, the three positions (a), (b) and (c) represented in FIG. 1 respectively relating to a weak field (less than the field necessary to be able to start to orientate the molecules), a medium field and a strong field.
By changing the orientation of said molecules, the average optical index experienced by a light beam passing through the medium can thus be varied. It is possible to use a non-twisted nematic with positive or negative dielectric anisotropy integrated into a cell, more precisely having an entry polarizer and two substrates, one of which, by virtue of a prior surface condition, makes it possible to constrain the molecules in an initial state.
Such a device operates with polarized light and, as the field increases, the liquid-crystal molecules straighten and the light experiences an index intermediate between the ordinary and extraordinary indices.
In order to modify the phase of unpolarized light, it is necessary to carry out this phase-shifting operation on the two components of the electric field, to which end it is possible to superpose two devices, as described in the article: Polarization-independent liquid crystal phase modulator using a thin polymer-separated double-layered structure, Yi-Hsin Lin, Hongwen Ren, Yung Hsun Wu, Zhibing Ge and Shin-Tson Wu: College of Optics and Photonics, University of Central Florida, Orlando, Fla. 32816 and Yue Zhao and Jiyu Fang: Advanced Materials Processing and Analysis Center and Department of Mechanical, Materials, or to use a quarter-wave plate and a mirror and thereby make the light pass two times through the same device, thus acting on the two components of the electric field as described in the article: Liquid-crystal phase modulator for unpolarized light by Gordon D. Love APPLIED OPTICS/Vol. 32, No. 13/1 May 1993.
In the first case, the superposition poses significant parallax problems and therefore does not permit superposition of matrices having small pixels. In the second case, the use of a reflector in the optical path makes it a reflection device, which therefore cannot be integrated into conventional optics (zoom optics for an imager, for example).
Liquid-crystal cells of the twisted nematic or cholesteric type, in which the molecules are not arranged exactly parallel to one another but adopt a helical configuration, are furthermore known. If a section of the structure is taken in a plane perpendicular to the z axis of the spiral, the distortion of the molecules in the plane is similar to that of a nematic but the privileged orientation direction of the molecules turns slowly when moving along the z axis. A periodic helical structure along the z direction perpendicular to the plane of the layers is thus obtained. Depending on the illumination wavelength and the pitch of the helix, such structures may behave partially as a mirror if the following condition is satisfied: p=λ/n with λ being the wavelength of the wave and n being the average index of the liquid-crystal medium.
In general, in order to obtain a liquid-crystal cell of the twisted nematic TN or cholesteric type, it is conventional to use two transparent substrates assembled to form a cavity between them, in which the liquid-crystal molecules are incorporated.
The rubbing operations known to the person skilled in the art, which are carried out on the two substrates, and the assembly of the latter are such that there is a so-called twist angle between the rubbing axes of the two alignment layers. For example, this twist angle may be 90°. It may also be less, for example 80°, or more, for example 280°, typically for so-called super-twisted nematics STN.
In general, liquid crystals of the cholesteric type become orientated spontaneously in a privileged direction n in space, which is called the director. When an electric field applies constraints in a certain orientation, the molecules tend to return to this state in response to a deformation.
In the context of a phase device, when light passes through these cholesteric liquid crystals in the OFF state, if the pitch of the liquid crystal is rapid enough, the latter has only a low rotating power on the light which then “sees” an average index known to be approximately no+½Δn, where no is the ordinary index of the liquid crystal and Δn is the difference between the extraordinary and ordinary indices (ne−no).
Moreover, for an arbitrary liquid-crystal thickness and a given pitch of the cholesteric, further to the low rotating power there is a change in the ellipticity of the light which passes through the device.
For example, linearly polarized light passing through the device is slightly elliptical when it leaves the device, the degree of ellipticity depending on the azimuth between the entry director of the liquid crystal, the direction of the polarization upon entry and also the wavelength.
This ellipticity could have been obtained in the same way by passing through a simple birefringent plate placed at 45° to the polarization in question: the cholesteric liquid crystal is then referred to as having a residual birefringence, the value of this birefringence being that of the equivalent plate defined above.